The present invention relates to magnetoresistive (MR) sensing mechanisms, which may for example be employed in information storage systems or measurement and testing systems.
FIG. 1 shows a prior art spin valve (SV) sensor 20 that may be used in a head of a disk drive. The sensor 20 has a magnetic pinning structure 22 that may be a permanent magnet, an antiferromagnetic material, or a synthetic antiferromagnetic structure. Pinning structure 22 may include plural layers, as is well known. For example, a synthetic antiferromagnetic structure includes a pair of ferromagnetic layers sandwiched about a very thin exchange coupling layer of ruthenium (Ru), iridium (Ir) or rhodium (Rh). The pinning structure 22 functions to pin or set the magnetization of an adjoining first ferromagnetic layer 25, which may be termed the pinned layer. Adjoining the pinned layer 25 is an electrically conductive spacer layer 27, which is adjoined by a second ferromagnetic layer 29 that may be termed the free layer.
The sensor 20 may be formed on a wafer substrate with thousands of other sensors. For the situation in which sensor 20 has the pinned layer 22 formed prior to the free layer 29, sensor 20 may be termed a bottom SV sensor. A SV sensor in which the order is reversed, with a free layer formed prior to the pinned layer, may be termed a top SV sensor.
After formation of the pinned and free layers as described above, the sensor 20 is separated from other sensors on the wafer by ion beam etching (IBE) or similar removal techniques, and chromium (CR) layers 30 are formed, followed by cobalt-chrome-platinum (CoCrPt) bias layers 33 and lead layer 35. The Cr layers provide a surface that encourages growth of bias layers 33 as a permanent or “hard” magnet. The bias layers 33 provide a magnetic field to the adjacent edges of the free layer, to reduce magnetic instability in those edges that could otherwise result in noise.
In operation, the leads 35 provide a sense current that flows along the conductive layer 27, with the resistance to that flow dependent upon the relative magnetization directions of the free 29 and pinned 27 layers. The magnetization direction of the free layer 29 is designed to change due to magnetic fields from a storage medium such as a disk, while the magnetization direction of the pinned layer 29 remains constant, so that the change in current or voltage of the leads 35 indicates the magnetic field direction of the medium.
FIG. 2 shows another type of bottom SV sensor 50, for which the pinning structure 22, pinned layer 25, conductive layer 27 and free layer 29 are essentially the same as described previously. Sensor 50 has a pair of antiferromagnetic layers 52, however, overlapping free layer 29 in order to pin edges of the free layer to reduce magnetic instability in those edges that could otherwise result in noise. Leads 55 cause current to flow through conductive layer 27, with the resistance to that current indicating the magnetic fields felt by the sensor.
The antiferromagnetic layers 52 are formed by IBE or similar trimming that may remove a small amount of the free layer 29, which can create additional edge effects in the free layer at the edge of the antiferromagnetic layers 52. It is also possible to have a hard bias layer overlapping the free layer, however, in this case a seed layer of Cr that is formed to encourage appropriate growth of the hard bias layer interferes with the coupling between the hard bias layer and the free layer.